The present invention relates to an optical communication system provided with an optical fiber amplifier and, more particularly, to an optical communication system provided with an optical fiber amplifier constituted of a rare-earth-doped fiber which is doped with a rare earth element.
Optical amplifiers capable of amplifying an optical signal directly, without using an electric circuit, are being intensively studied in many research institutions as a key device in the optical communication system of the future because of their bit-rate-free characteristic, ability to provide large capacity, and capability of amplifying multiple channels en bloc. As optical communication systems including such optical amplifiers, there are proposed those that employ the optical amplifier as an optical power booster for compensating for branching or insertion loss and increasing the transmission power, those that use the optical amplifier as an optical preamplifier for improvement of reception sensitivity, those that use the optical amplifier as an optical repeater, whereby miniaturization of the repeater is achieved and reliability of the repeater is enhanced, and so on. Research is being conducted for optimized arrangement of such systems.
Optical amplifiers, which are the objects of the research conducted so far, are broadly classified into: (a) amplifiers using an optical fiber doped with a rare earth element such as Er, Nd, and Yb (hereinafter referred to as "rare-earth-doped fiber", wherein this term covers a wide range of waveguide structures including a waveguide doped with a rare earth element); (b) semiconductor laser type amplifiers; and (c) amplifiers which make use of the non-linearity within an optical fiber. Of these, the optical amplifier (a) above, i.e., one that uses a rare-earth-doped fiber, has a number of advantageous characteristics, such as polarization independence, low noise production, and a small loss at its coupling with the transmission line.
When an optical amplifier is used as an optical repeater, a supervisory control function for the optical amplifier is indispensable. As a supervisory system applicable to the optical amplifier (b) above, i.e., the semiconductor laser type, there is known a system disclosed, for example, in Ellis, A. D. et al.: Supervisory System for Cascaded Semiconductor Laser Amplifier Repeaters, Electron. Lett., Vol 25, No. 5, pp. 309-311 (2nd March 1989). Since in this system the injection current to the semiconductor laser type optical amplifier is detected therein, the same system cannot be applied to the optical communication system provided with an optical fiber amplifier. In other words, there is no prior art supervisory control system which is suitable for optical fiber amplifiers.